The invention relates to a method for blowing steam against a paper web, in which method the steam is blown by a steam box having several profiling chambers in the cross direction of the paper machine, and the steam blown by the profiling chambers is used to control the cross-profile of the paper web, and each profiling chamber blows steam to its own effective area on the paper web.
Further, the invention relates to a steam box of a paper machine having several profiling chambers in the cross direction of the paper machine, and the profiling chambers are arranged to control the cross-profile of the paper web in such a manner that each profiling chamber is arranged to blow steam to its own effective area on the paper web.
Paper machines use steam boxes which are used to blow steam against the paper web. The steam box is installed perpendicular to the direction of travel of the paper web in such a manner that it extends across the entire width of the web. By means of steam boxes it is, for instance, possible to increase the capacity of the paper machine. The steam box can, for instance, be arranged in the press and/or wire section of the paper machine, in which case the steam box can, for instance, be used to increase the temperature of the paper web, and, by means of press nips and dewatering equipment, to drain the warmed water more easily and thus increase the dry content and improve the drying capacity of the dryer section. Further, the steam box can be used to correct the moisture profile of the paper web, if necessary. Correcting the moisture profile is done by applying a different amount of steam at different locations to the paper web in its cross direction. The steam box is in its cross direction usually divided into several chambers, or profiling chambers, which each have a separate steam supply. Steam boxes can be used in many other parts of the paper machine, such as the dryer section, calendering, and paper converting machines.
Finnish patent 101 727 discloses a steam box which has a steam chamber for blowing steam towards the paper web and a suction zone extending across the entire paper web in the output direction of the machine, the suction zone being located after the steam chamber in the direction of travel of the paper. The purpose of the suction zone is to prevent the spreading of steam spray to the paper machine surroundings, i.e. to suck the excess steam away. After the suction zone, in the direction of travel of the web, there is a blow nozzle for slowing down the steam flow along the web past the suction zone. It is, however, not possible to control the cross-profile of the paper web with the described solutions.
U.S Pat. No. 4,662,398 discloses a steam box of a paper machine, in which steam is fed through a steam supply pipe to a steam space inside the steam box. The steam box is in the cross direction of the web divided into consecutive chambers, and steam is led to each chamber from the steam space through adjustable valves in each chamber. One wall of each chamber is a nozzle plate through which steam affects the paper web. By means of the valves, it is possible to adjust the amount of steam flowing into each chamber specifically for each chamber and thus, by means of the profiling chambers in question, it is possible to control the cross-profile of the paper web.
Finnish publication 963 583 discloses a steam box of a paper machine, in which the valves controlling the amount of steam are arranged outside the steam box, and steam is led into steam chambers through pipes specific for each chamber. The purpose is to make the profiling chambers as small as possible and thus, to direct the profiling effect to as many locations as possible in the cross direction of the paper web. However, the steam pipes cause rather high flow and heat losses, which means that controlling the cross-profile of the paper web is very difficult and uncertain by means of the steam box in question. Further, the high number of chambers makes the control very complex. As a whole, the solution is quite expensive and difficult in structure.
The problem with trying to achieve a profiling effect on the paper web by means of a steam box is that the steam blown by a profiling chamber also spreads in the cross direction of the paper web to the effective area of the steam blown by the adjacent profiling chamber. To improve the profiling effect, the number of profiling chambers has been increased. However, the width of one profiling chamber in the cross direction of the machine then becomes quite small. The width of one profiling chamber in the cross direction of the machine can be 50 mm, for instance. The distance of the steam box from the surface of the paper web can be 10 to 30 mm, for instance. In such a case, the steam blown by a profiling chamber may even affect almost the entire effective area of the adjacent profiling chambers, and the desired profiling effect on a specific web location is not achieved. This makes controlling the profiling effect of the steam box very difficult, i.e. the profiling effect as a whole is unsatisfactory.
It is an object of the present invention to provide a method and a steam box, by means of which it is possible to improve the profiling effect of the steam box.
The method of the invention is characterized in that a gas flow is provided between the effective areas of the profiling chambers to prevent the steam blown by a profiling chamber from affecting the effective area of the steam blown by the adjacent profiling chamber.
Further, the steam box of the invention is characterized in that the steam box has intermediate nozzles between the profiling chambers for providing a gas flow in such a manner that the gas flow is arranged to prevent the steam blown by a profiling chamber from affecting the effective area of the steam blown by the adjacent profiling chamber.
The essential idea of the invention is that the steam box has in cross direction of the paper machine several profiling chambers, and the steam blown from the profiling chambers is used to control the cross-profile of the paper web. A gas flow is provided between adjacent profiling chambers to prevent the steam blown by a profiling chamber from affecting the effective area of the steam blown by the adjacent profiling chamber. The idea of a preferred embodiment is that steam supplied by the profiling chambers is sucked from between the profiling chambers. The idea of a second preferred embodiment is that intermediate nozzles are arranged between the profiling chambers, and they are used to blow gas between the effective areas of the steam blown by the profiling chambers. The idea of a third preferred embodiment is that the gas blown between the effective areas of the steam blown by the profiling chambers is air. The idea of a fourth preferred embodiment is that steam is blown between the effective areas of the steam blown by the profiling chambers. The idea of a fifth preferred embodiment is that the blowing rate of the gas is arranged to be higher than the rate of the steam blown by the profiling chambers. The idea of a sixth preferred embodiment is that the temperature of the gas blown between the effective areas of the steam blown by the profiling chambers is substantially equal to the temperature of the steam blown by the profiling chambers.
The invention provides the advantage that it is possible to slow down and thus reduce the movement of the steam blown by the profiling chamber in the cross direction of the paper machine, i.e. to prevent the spreading of the steam in the cross direction of the paper machine. This means that the effective area of the steam blown by different profiling chambers of the steam box can be directed exactly to the desired location of the web, i.e. the profiling effect of the steam box improves. By sucking steam away from between the profiling chambers, it is possible to easily improve the profiling effect without affecting the paper web properties between the profiling chambers. On the other hand, blowing gas between the effective areas of the steam blown by the profiling chambers is simple to implement. Blowing air between the effective areas of the steam blown by the profiling chambers is quite effective, because air binds steam quite efficiently. Blowing steam is easy to arrange, because steam is readily available from the steam box. By arranging the temperature of the blown gas to be the same as the temperature of the steam blown by the profiling chamber means that the effect of the gas blown between the effective areas of the steam blown by the profiling chambers does substantially not differ from the effect of the steam blown by the profiling chambers on the paper web.
In this description, the term xe2x80x98paperxe2x80x99 refers to the different embodiments of paper, such as board, tissue, and pulp.